Helicobacter pylori, a Gram-negative microaerophilic rod, colonizes the human stomach infecting nearly one-half of the world's population. The organism has been classified as a class I carcinogen and plays a major etiological role in human gastritis, peptic ulcer disease, and gastric carcinoma. Humans are the only significant reservoir for H. pylori, despite the hostile environment of the gastric mucosa. Therefore, a unique balance must be established in order to permit long-term survival of both the bacterium and its human host. The outer surface of H. pylori and other Gram-negative bacteria consists primarily of lipopolysaccharide that is composed of three distinct structural domains: O-anUgen, core, and lipid A. Lipid A, also known as endotoxin, is the membrane anchor of lipopolysaccharide and is the portion of the molecule responsible for its endotoxicity. The modification of the lipid A structure is an emerging theme in Gram-negative bacteria that has been reported to contribute to virulence and bacterial pathogenesis. It has been suggested that the LPS structure of H. pylori has evolved to aid the bacterium in evading the host innate immune system, thereby prolonging bacterial infection. Our overall objective is to unravel the molecular mechanisms by which H. pylori modify the structure of their LPS using biochemical and molecular biological approaches. More specifically, we intend to identify novel enzymes and the genes encoding them that are responsible for modification of the lipid A and core domains of H. pylori IPS. The specific aims of the current proposal are: (i) characterization of modifications to the inner core region of H. pylori LPS; (ii) characterization of modifications to the phosphate groups of H. pylori lipid A; (iii) characterization of acyl-chain rearrangement of H. pylorilipid A. Humans are constantly exposed to a variety of microorganisms having the potential to cause disease. One such organism is a bacterium known as Helicobacter pylori that lives in the human stomach, a niche where no other microorganism is known to thrive. Infection with H. pylori is a predisposing factor for peptic ulcer disease and stomach cancers. The current proposal will help determine howthese unique bacteria are able to survive in the human stomach leading to a disease state.